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(Week ending May 5, 2001-Week 18)

The next report will be available in two weeks (May 25, 2001).

The following information may be quoted:

Synopsis: During week 18 (April 29 � May 4, 2001)*, none of the specimens tested by WHO and NREVSS laboratories were positive for influenza. The proportion of patient visits to sentinel physicians for influenza-like illness was within baseline levels of 0 % to 3 % in the United States overall and in all 9 surveillance regions. The proportion of deaths attributed to pneumonia and influenza was 6,8 %. This percentage is below the epidemic threshold for this time of year. Ten state and territorial health departments reported sporadic influenza activity** and 38 reported no influenza activity.

For the current season, the overall national percentage of respiratory specimens positive for influenza appears to have peaked at 24 % at the end of January (week 4). During the past 3 seasons, the peak percentages of respiratory specimens positive for influenza viruses have ranged from 28 % to 33 %. For this season, the percentage of patient visits to sentinel physicians for influenza-like illness appears to have peaked at 4 % in mid to late January. During the past 3 seasons, the peak percentages for such visits ranged between 5 % and 6 %.

U.S. World Health Organization (WHO) and National Respiratory And Enteric Virus Surveillance System(NREVSS) Collaborating Laboratory Reports *: During week 18, WHO and NREVSS laboratories reported 347 specimens tested for influenza viruses. No influenza isolates were identified. During the past 3 weeks (weeks 16-18), 31 (2 %) of the specimens tested for influenza were positive and all were influenza B viruses. Less than 5 % (range 0 % to 3 %) of specimens tested for influenza during the past 3 weeks in each of 9 surveillance regions*** were positive.

Since October 1, WHO and NREVSS laboratories have tested a total of 83 589 specimens for influenza viruses, and 9 781 (12 %) were positive. Of the 9 781 isolates identified, 5 289 (54 %) were influenza type A and 4 492 (46 %) were influenza type B. Two thousand and eighty-five (39 %) of the 5 289 influenza A viruses identified have been subtyped; 1 985 (95 %) were A (H1N1) and 100 (5 %) were A (H3N2). Influenza A viruses have predominated (range 52 % to 63 %) in the East South Central, West South Central, West North Central, East North Central, and South Atlantic regions, and influenza B viruses have predominated (range 53 % to 63 %) in the Mountain, Pacific, and Mid-Atlantic regions. The New England region has reported approximately equal numbers of influenza A and influenza B isolates this season.

 Influenza Virus Isolated View Chart Data

Antigenic Characterization of Viral Isolates: CDC has antigenically characterized 685 influenza viruses received from U.S. laboratories since October 1. Of the 353 influenza A (H1N1) isolates that have been characterized, 334 (95 %) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000-01 influenza vaccine, and 19 (5 %) were similar to A/Bayern/07/95. Although A/Bayern-like viruses are antigenically distinct from the A/New Caledonia-like viruses, the A/New Caledonia/20/99 vaccine strain produces high titers of antibody that cross-react with A/Bayern/07/95-like viruses. Of the 30 influenza A (H3N2) viruses that have been characterized, all were antigenically similar to the vaccine strain A/Panama/2007/99. Of the 302 influenza B viruses characterized, 29 (10 %) were similar to the vaccine strain, B/Beijing/184/93, and 273 (90 %) were more closely related antigenically to the B/Sichuan/379/99 reference strain. It should be noted that the B/Sichuan virus exhibits cross-reactivity with the vaccine strain.

Pneumonia and Influenza (P&I) Mortality: During week 18, the percentage of all deaths due to P&I as reported by the vital statistics offices of 122 U.S. cities was 6,8 %. This percentage is below the epidemic threshold of 8,2 % for week 18.

Pneumonia And Influenza Mortality 





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Influenza Morbidity Reports from U.S. Sentinel Physicians*: During week 18, less than 1 % of patient visits to U.S. sentinel physicians were due to influenza-like illness (ILI). The percentage of patient visits for ILI was within baseline levels of 0 % to 3 % in all 9 surveillance regions.

Bar Chart for Influenza-like Illness View Chart Data

Influenza Activity as Assessed by State and Territorial Epidemiologists**: Influenza activity was reported as sporadic in 10 states (Arizona, Florida, Hawaii, Kentucky, Missouri, New Mexico, Ohio, Tennessee, Virginia, and West Virginia). Thirty-eight states reported no influenza activity and 2 states did not report.

Usmap for Weekly Influenza Activity

* Reporting is incomplete for this week. Numbers may change as more reports are received.

** Influenza activity is defined as influenza-like illness and/or culture-confirmed influenza.

*** Surveillance Regions: New England (Connecticut, Maine, Massachusetts, New Hampshire, Vermont, Rhode Island); Mid-Atlantic (New Jersey, New York City, Pennsylvania, Upstate New York); East North Central (Illinois, Indiana, Michigan, Ohio, Wisconsin); West North Central (Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota); South Atlantic (Delaware, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, Washington, D.C., West Virginia); East South Central (Alabama, Kentucky, Mississippi, Tennessee); West South Central (Arkansas, Louisiana, Oklahoma, Texas); Mountain (Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, Wyoming); Pacific (Alaska, California, Hawaii, Oregon, Washington)

Report prepared: Report prepared: May 10, 2001

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